The faster potassium-lowering effect of high dialysate bicarbonate concentrations in chronic haemodialysis patients

Background. Hyperkalaemia is common in patients with advanced renal disease. In this double-blind, randomized, three-sequence, crossover study, we compared the effect of three dialysate bicarbonate concentrations ([ \(\mathrm{HCO}_{3}^{{-}}\) ]) on the kinetics of serum potassium (K+) reduction during a conventional haemodialysis (HD) session in chronic HD patients. Methods. We studied eight stable HD patients. The choice of dialysate [ \(\mathrm{HCO}_{3}^{{-}}\) ] followed a previously assigned treatment protocol and the [ \(\mathrm{HCO}_{3}^{{-}}\) ] used were low bicarbonate (LB; 27 mmol/l), standard bicarbonate (SB; 35 mmol/l) and high bicarbonate (HB; 39 mmol/l). Polysulphone dialysers and automated machines provided blood flow rates of 300 ml/min and dialysis flow rates of 500 ml/min for each HD session. Blood samples were drawn at 0 (baseline), 15, 30, 60 and 240 min from the arterial extracorporeal line to assess blood gases and serum electrolytes. In three of the eight patients, we measured serum K+ 1 h post-dialysis as well as K+ removal by the dialysis. The same procedures were followed until the completion of the three arms of the study, with a 1 week interval between each experimental arm. Results. Serum K+ decreased from 5.4±0.26 (baseline) to 4.96±0.20, 4.90±0.19, 4.68±0.13 and 4.24±0.15 mmol/l at 15, 30, 60 and 240 min, respectively, with LB; from 5.38±0.21 to 5.01±0.23, 4.70±0.25, 4.3±0.15 and 3.8±0.19 mmol/l, respectively, with SB; and from 5.45±0.25 to 4.79±0.17, 4.48±0.17, 3.86±0.16 and 3.34±0.11 mmol/l, respectively, with HB (P<0.05 for high vs standard and low [ \(\mathrm{HCO}_{3}^{{-}}\) ] at 60 and 240 min). The decrease in serum K+ correlated with the rise in serum [ \(\mathrm{HCO}_{3}^{{-}}\) ] in all but LB (P<0.05). Potassium rebound was 3.9±10.2%, 5.2±6.6% and 8.9±4.9% for LB, SB and HB dialysates, respectively (P = NS), while total K+ removal (mmol/dialysis) was 116.4±21.6 for LB, 73.2±12.8 for SB and 80.9±15.4 for HB (P = NS). Conclusions. High dialysate [ \(\mathrm{HCO}_{3}^{{-}}\) ] was associated with a faster decrease in serum K+. Our results strongly suggest that this reduction was due to the enhanced shifting of K+ from the extracellular to the intracellular fluid compartment rather than its removal by dialysis. This finding could have an impact for those patients with life-threatening pre-HD hyperkalaemia.